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19 February, 2016

RMAN : Unused Block Compression and Null Block Compression

To quote the 11.2 documentation, "Although it is referred to as block compression, it might be helpful to think of block compression as block skipping". 11.2 differentiates between Unused Block Compression and Null Block Compression.UPDATE 28-Feb-16 : Please note the list of 5 pre-requisites for Unused Block Compression.

In response to a community thread, here is a quick demo to see both Null Block Compression and Unused Block Compression.

Aha ! BackupSet 31 is now 1.03MB only. So, Unused Block Compression has kicked in ! The blocks that belonged to that table are yet formatted and in prior versions that did not have Unused Block Compression would still be backed up even though the table has been dropped.
(Note : I did not shrink the datafile, it is still 100MB).